Currently, most of the visual and audio equipments exhibit a plurality of terminals or plugs for receiving external device to input or output signal. The popular application is the micron phone-jacks, especially for the usage for the computer system. Such terminal not only has small size but also occupied a small space. In order to detect whether the plug is set on the jack or not, a typical phone-jack with spring switch is used. Please referring to FIG. 1a, it shows the prior art structure for the detecting purpose. The phone-jack includes connecting terminals 1 and 2 that are referred to the spring switch. Connecting terminals 3 and 4 are the signal contacts and the connecting terminal 5 are the common reference contact for the signal contacts 3 and 4. The spring switch is typically designed on the status of open when a device is plugged into the jack, otherwise the status is short circuit, as shown in FIG. 1b. It illustrates when a device is plugged into the phone-jack. In the prior art, a resistor and a reference voltage VDD are added into the structure (please refer to FIGS. 1a and 1b). The status is detected by the “Readback” node. In the case, if there is no any device connects to the jack 1, the aforementioned spring switch is coupled to the “ground”, thereby the “ground voltage” is detected on the node of “Readback”. On the contrary, a VDD is detected on the node of “Readback” by coupling the 10K ohms resistor. If the statuses are changed to inverse to the status mentioned above, namely, the status is short circuit when a device is plugged into, the status of the “Readback” node are inverse to the mentioned statuses. The system will obtain the phone-jack 1 status by detecting the “Readback” node. The prior detecting method needs a “Readback” node for each phone-jack. Thus, the number of the “Readback” node is increased along with the increasing number of the phone-jack.
If the detecting circuits are integrated in a single chip, a plurality of the “Readback” node must be formed in such single chip. The situation raises the cost of the manufacture. The present invention discloses a structure of detecting the status by using single bit generated by a resistor network.
The present invention is suitable for the computer system, it also can be used in other system.